Selected genes in Drosophila melanogaster are being studied in an effort to understand their organization and regulation through the analysis of mutations that upset regulatory functions. A locus of particular interest is white, which controls one of the steps in the pigmentation of the eyes, Malpighlian tubules and testis sheath in Drosophila. The molecular analysis of mutants shows that a large proportion of spontaneous changes result from the insertion/deletion of transposable elements. We are studying mutations that cause mosaic expression of the white locus. The original mutation of this series resulted from the insertion of the transposon BEL into the large intron of white. This mutant wzm, produces a mottle-eye phenotype only when combined with the mutation z1, otherwise it has a wild-type phenotype. wzm is mildly unstable and has produced a series of other alleles including a transposition of an X chromosome segment containing white into the third chromosome. In the new position, the white locus produces a wild-type phenotype except when the transposition is homozygous and the z1 mutation is present. That genotype produces a mosaic eye-color that is nonautonomous and nonclonal in expression, marking a dramatic change from the autonomous, clonal patterns seen in the non-transposed wzm and its derivatives. The molecular analyses of these mutant alleles are being done to determine the basis for the modified expression of white and its interaction with z1. We have also resolved the basis for four unexpected regularly-occurring asymmetrical recombination products recovered from wbf/wa heterozygotes. The transposon roo (also known as B104) is found within the white locus and approximately 30 kb proximal to white in the wbf chromosome. roo had also invaded copia at the wa site in the wa chromosome. The three copies of roo pair and undergo exchange in two separate configurations, resulting in two sets of reciprocal duplication and deficiency products. This defines another important way that transposon can reshape eukaryotic genomes.